To date, more than 200 microRNAs have been described in humans; however, the precise functions of these regulatory, non-coding RNAs remains largely obscure. One cluster of microRNAs, the mir-17-92 polycistron, is located in a region of DNA that is amplified in human B-cell lymphomas 1 . Here we compared B-cell lymphoma samples and cell lines to normal tissues, and found that the levels of the primary or mature microRNAs derived from the mir-17-92 locus are often substantially increased in these cancers. Enforced expression of the mir-17-92 cluster acted with c-myc expression to accelerate tumour development in a mouse B-cell lymphoma model. Tumours derived from haematopoietic stem cells expressing a subset of the mir-17-92 cluster and c-myc could be distinguished by an absence of apoptosis that was otherwise prevalent in c-myc-induced lymphomas. Together, these studies indicate that non-coding RNAs, specifically microRNAs, can modulate tumour formation, and implicate the mir-17-92 cluster as a potential human oncogene.MicroRNAs (miRNAs) have emerged relatively recently as a new class of small, non-coding RNAs that regulate gene expression. Nascent primary miRNA transcripts (pri-miRNAs) are processed sequentially by two RNase III enzymes, Drosha and Dicer 2,3 , to yield mature miRNAs, ranging from 18 to 24 nucleotides (nt) in length. miRNAs are incorporated into the RNA interference (RNAi) effector complex, RISC, and target specific messenger RNAs Reprints and permissions information is available at npg.nature.com/reprintsandpermissionsCorrespondence and requests for materials should be addressed to G.J.H. (hannon@cshl.edu) or S.M.H. (hammond@med.unc.edu). * These authors contributed equally to this work.Supplementary Information is linked to the online version of the paper at www.nature.com/nature.Microarray data have been deposited in NCBI-GEO under accession numbers GSM45026-GSM45065 and GSE-2399.The authors declare no competing financial interests. -6 and miR-273 (refs 9, 10). Bantam stimulates cell growth and prevents apoptosis in Drosophila 11 , and miR-181 potentiates B-cell differentiation in mammals 12 . These findings, in combination with computational target predictions, are consistent with miRNAs regulating a broad spectrum of physiological and developmental processes. HHS Public AccessMicroarray-based expression studies have indicated specific alterations in human miRNA expression profiles that correlate with particular tumour phenotypes (J.M.T. and S.M.H., unpublished data). Among those that show altered expression, the mir-17-92 cistron is located at 13q31, a genomic locus that is amplified in cases of diffuse large B-cell lymphoma, follicular lymphoma, mantle cell lymphoma, primary cutaneous B-cell lymphoma and several other tumour types 1,13 . There are only two annotated genes in the epicentre of this amplicon, c13orf25 and GPC5. Previous studies have shown that c13orf25 is the only one of the two genes for which increased expression correlates with the presence of the amplicon 1 . The...

Although cancer arises from a combination of mutations in oncogenes and tumour suppressor genes, the extent to which tumour suppressor gene loss is required for maintaining established tumours is poorly understood. p53 is an important tumour suppressor that acts to restrict proliferation in response to DNA damage or deregulation of mitogenic oncogenes, by leading to the induction of various cell cycle checkpoints, apoptosis or cellular senescence 1,2 . Consequently, p53 mutations increase cell proliferation and survival, and in some settings promote genomic instability and resistance to certain chemotherapies 3 . To determine the consequences of reactivating the p53 pathway in tumours, we used RNA interference (RNAi) to conditionally regulate endogenous p53 expression in a mosaic mouse model of liver carcinoma 4,5 . We show that even brief reactivation of endogenous p53 in p53-deficient tumours can produce complete tumour regressions. The primary response to p53 was not apoptosis, but instead involved the induction of a cellular senescence program that was associated with differentiation and the upregulation of inflammatory cytokines. This program, although producing only cell cycle arrest in vitro, also triggered an innate immune response that targeted the tumour cells in vivo, thereby contributing to tumour clearance. Our study indicates that p53 loss can be required for the maintenance of aggressive carcinomas, and illustrates how the cellular senescence program can act together with the innate immune system to potently limit tumour growth.p53 mutations are common in human liver cancer 6 , which is typically highly aggressive and resistant to non-surgical therapies. To determine the requirement for p53 loss in the maintenance of such carcinomas, we used reversible RNAi 7 to control p53 in a chimaeric liver cancer mouse model (Fig. 1a) 4,5 . Purified embryonic liver progenitor cells (hepatoblasts) were transduced with retroviruses expressing oncogenic ras (HrasV12), the tetracycline transactivator protein tTA ('tet-off') and a tet-responsive p53 miR30 design short hairpin RNA (shRNA; Supplementary Fig. 1a) 7,8 , and seeded into the livers of athymic nude mice following intrasplenic injection 4,5 . To facilitate in vivo imaging, the oncogenic ras retrovirus co-expressed green fluorescent protein (GFP) and, in some experiments, hepatoblasts were also co-transduced with a luciferase reporter.p53 expression was efficiently suppressed in the absence of doxycycline (Dox) and rapidly restored following Dox addition ( Supplementary Fig. 1b, c). On transplantation into the livers of recipient mice, hepatoblast populations co-expressing Ras and the conditional p53 shRNA rapidly produced invasive hepatocarcinomas in the absence of Dox (Fig. 1b), whereas cells expressing each vector alone did not (data not shown). These tumours were GFP-positive and, if expressing luciferase, could be visualized externally by bioluminescence imaging (Fig. 1b).

Cells enter senescence, a state of stable proliferative arrest, in response to a variety of cellular stresses, including telomere erosion, DNA damage, and oncogenic signaling, which acts as a barrier against malignant transformation in vivo. To identify genes controlling senescence, we conducted an unbiased screen for small hairpin RNAs that extend the life span of primary human fibroblasts. Here, we report that knocking down the chemokine receptor CXCR2 (IL8RB) alleviates both replicative and oncogene-induced senescence (OIS) and diminishes the DNA-damage response. Conversely, ectopic expression of CXCR2 results in premature senescence via a p53-dependent mechanism. Cells undergoing OIS secrete multiple CXCR2-binding chemokines in a program that is regulated by the NF-kappaB and C/EBPbeta transcription factors and coordinately induce CXCR2 expression. CXCR2 upregulation is also observed in preneoplastic lesions in vivo. These results suggest that senescent cells activate a self-amplifying secretory network in which CXCR2-binding chemokines reinforce growth arrest.

Mad2 is an essential component of the spindle checkpoint that blocks activation of Separase and dissolution of sister chromatids until microtubule attachment to kinetochores is complete. We show here that overexpression of Mad2 in transgenic mice leads to a wide variety of neoplasias, appearance of broken chromosomes, anaphase bridges, and whole-chromosome gains and losses, as well as acceleration of myc-induced lymphomagenesis. Moreover, continued overexpression of Mad2 is not required for tumor maintenance, unlike the majority of oncogenes studied to date. These results demonstrate that transient Mad2 overexpression and chromosome instability can be an important stimulus in the initiation and progression of different cancer subtypes.

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